So at least sites that spy on their users will have a worse UX than before. That is an improvement, though a small one.

Speak for yourself. In my opinion, the ability to count how many people click on which search results, and rank results higher if they get a lot of clicks (excluding cases where the user then pressed back and picked another result), seems like a good way to boost result quality without having much impact on privacy.

Full disclosure – I also allow Google to track my search history for personalization, but I think there are many good reasons to dislike that, I understand why people turn it off, and while I don't, I'd prefer a personalization method that doesn't require such granular data.

This kind of tracking might be worth it, given informed consent by the user. If the user does not consent, it is spying. Especially if you have to bypass intentional privacy protections, in this case the disabling of the ping feature.

Or Firefox will end up with a worse UX than before. I'm pretty sure the spying sites offer the cumbersome solution only to browsers that have ping disabled.

What you need is a browser that claims to honour ping but doesn't.

I do not think Mozilla can keep that secret.

Neither the code nor the server has to know which browser you are.